Industries generate various forms of wastes as part of production, manufacturing and pollution control operations. Among various wastes, sludges are one of the most common forms of wastes. Examples of industry groups and associated sludge sources include the following:
Chemical--wastes and wastewater sludges PA1 R.sub.e =Evaporation potential for mixed and aerated sludge on a paved bed (lb/year.ft.sup.2), PA1 K.sub.e =A reduction factor, a typical value is 0.6, and PA1 Ep=Pan evaporation rate (ft/year). PA1 Re=Evaporation potential for mixed and aerated sludge on a pave bed (in/year) PA1 Ep=Pan evaporation rate (in/year).
Petrochemical--filter clay, API separator bottom sludge, wastewater sludge PA0 Aluminum--red mud, brown mud, gypsum PA0 Power plants--flue gas desulfurization sludge, flyash, gypsum, cooling water mud PA0 Metal Finishing/Plating--grit sedimentation, acid neutralization and dissolved metal precipitation sludges PA0 Pulp/paper--lime mud, fiber sludge, waste clay, wastewater sludge PA0 Phosphate mining--clay slime PA0 Mining, metals, and minerals--mine and mill tailings PA0 Drinking water supply--alum and ferric coagulation sludges PA0 Food processing--food waste and wastewater sludges PA0 Textile, dye, pigment--dye wastes and wastewater sludges PA0 Sewer treatment plants--wastewater sludge PA0 Pharmaceutical--waste and wastewater sludges PA0 Plastics, rubber--waste and wastewater sludges PA0 Iron, steel, coke--primary, scrubber blowdown, wastewater sludges
Sludge wastes are very difficult to manage. They are too thick to discharge into natural water bodies, too soft to be built upon or to be placed in a landfill, too high in various chemicals to neglect and too large in quantity to manage by simple disposal. Sludge wastes are considered "unstable" because they tend to flow if not contained, are too soft to support any significant load, and deform excessively under loads. Generally, it is the high water content of sludge waste which makes it unstable. Most sludge wastes are not suitable for productive uses.
Some of the most common sludge management practices include 1) disposal in ponds, 2) mechanical dewatering, 3) solidification/stabilization, and 4) air drying.
Pond disposal is only an interim solution. When a disposal pond is full or further disposal is not allowed due to certain regulatory requirements, the sludge pond has to be closed, usually with a final cover installed over the pond to securely contain the waste.
Typical sludges in sludge disposal ponds contain 10 to 40% of solids by weight. A low solids content is characteristic of sludges containing biologic or amorphous salt/minerals with a low specific gravity. A high solid content is characteristic of sludges with plate-like or block-like particles of minerals and grit with a high specific gravity. A typical sludge having a solids content of 30% by weight has a solids content of only about 15% by volume because the solid part of the sludge has a higher specific gravity than water. Therefore, the majority (85%) of the sludge volume is water. This signifies the value of water removal from the sludge.
If a substantial portion, but not necessarily 100%, of this water is somehow removed, the sludge water content will be much lower, the sludge more stable, and the sludge volume much less. Water can be removed by various forms of mechanical dewatering such as filter press, belt press, centrifuge, vacuum filter, and others. Mechanical dewatering is most commonly used as the last step in a sludge generation process prior to landfill disposal. Less frequently, this process is also used as a stabilization step for closure of sludge ponds. Dewatered sludges are often stabilized further by a solidification/stabilization process. Unfortunately, mechanical dewatering is very expensive, costing typically between $25 and $70 per cubic yard of raw sludge, depending on the sludge characteristics, dewatering method and conditioning needs. At this cost, the advantages of lower water content and volume disappear. To be practical, the method of water removal and volume reduction should be economical.
To install a final cover and to minimize leachate migration into the environment, a sludge requires "stabilization," either physical stabilization (solidification) or chemical stabilization (fixation). Physical stabilization improves the physical properties of the sludge to support the final cover and construction equipment. Chemical stabilization "fixates" certain chemicals in the sludge to reduce the leachability of chemical constituents. Often, chemical stabilization also achieves physical stabilization and vice versa. This is why sludge or soil stabilization is often referred to as a solidification/stabilization (S/S) process, implying both physical stabilization and fixation.
The most common method of sludge stabilization is the solidification/stabilization (S/S) process in which various dry reagents are mixed with the sludge. Examples of reagents include cement, flyash, lime, lime kiln dust, cement kiln dust, slag, silicate, and other proprietary additives.
Three major effects of reagent stabilization are: (1) removal of excess water in the sludge by hydration; (2) hardening of the sludge/reagent mixture; and (3) fixation of inorganic chemicals, primarily metals, by various chemical reactions. With removal of water and hardening of the material, the stabilized wastes can support the final cover and cover installation equipment. With removal of water and fixation of certain chemical constituents, post-closure migration of chemicals through leachate is minimized.
This process has been widely applied to various sludge wastes for the closure of existing disposal ponds or for the treatment of sludges at the end of a waste generating process prior to landfill disposal. However, the S/S process increases the total volume and weight of the waste because a significant amount of reagent is added to treat the entire mass including water. An increase in waste volume or weight is undesirable in waste management. In addition, S/S is an expensive treatment method, costing from a low of $20 to a high of $100 or more per cubic yard depending on the waste properties, site conditions, and S/S performance requirements.
Air drying is another common method of sludge dewatering. The following three types of sludge drying beds have often been used to dewater municipal wastewater sludges: sand drying beds, drying lagoons and paved drying beds. Dilute liquid sludges with about 5% solids content are placed in these facilities and allowed to drain and to dry to a solids content between 25 and 40%. Dried sludge is removed by various equipment such as a front end loader or manually for small quantities. The removed sludge is hauled away for subsequent disposal such as landfilling or land application.
A description of sand drying beds is found in USEPA 1987, Section 6.2. Typical construction of a sand drying bed consists of a large area of a sand bed underlain by a layer of gravel and enclosed by concrete sidewalls. Sludge is applied on the sand bed, with a typical application depth of 8 to 18 inches. Water is drained by gravity into the sand bed and then into the gravel layer. Drain pipes embedded in the gravel layer remove water from the gravel layer. In addition to gravity drain, additional water is removed by evaporation. Since the majority of water is removed by gravity drain, the local climate is not critical for sand bed performance. One sludge drying cycle is four to twelve weeks, depending on climate and sludge conditioning.
Some sand beds are constructed with a greenhouse roof to avoid rewetting by rain (El-Ariny A.S., "Utilization of Solar Energy for Sludge Drying Beds," Journal of Solar Energy Engineering, Vol. 106, No. 3, Aug. 1984). Since some sand is also removed during sludge removal, sand should be replenished periodically. Sand bed performance may be enhanced by freezing the sludge or by applying a vacuum to the drain pipe.
A description of drying lagoons is found in USEPA, 1987, Section 6.2 and USEPA, 1974, Section 7.5. A drying lagoon is very similar to a sand bed except that the drainage is provided by the natural sand deposit at the bottom of the lagoon and the sidewall is an earthen dike. Sludge is applied to the lagoon in 24 to 48 inch depths and allowed to drain by gravity and to dry by evaporation. Lagoon drying is relatively inefficient, requiring several months to one year per cycle of drying, and therefore requires large land areas. Dewatered sludge is removed by mechanical equipment. Drained water enters the ground water body and degrades the ground water quality. Because of its impact on the ground water quality, this method is no longer a viable method of sludge dewatering.
Two primary problems exist with sand beds and drying lagoons: the drying rates are very slow after a surface crust is formed and the drying areas are not adequate to support sludge removal equipment. Drying beds constructed with a concrete or asphalt pavement solve these two problems. A description of paved drying beds is found in USEPA, 1987, Section 6.7.
The drying rate in a paved drying bed may be improved by breaking the surface crust and aerating the wet sludge with mechanical equipment. Various equipment can be used on top of the paved beds to aerate the sludge and to remove the dried sludge. However, this method sacrifices good bottom drainage, typical of sand beds and drying lagoons, due to the relatively impermeable underlying bed material such as asphalt and concrete.
The process of drying sludge by using a paved drying bed involves applying the sludge to the paved bed, decanting water after the settling of solids, and allowing the sludge to dry. After the surface crust is formed, the sludge is turned and aerated, to accelerate the drying process. The paved drying bed works best in warm, arid and semi-arid climates since water loss depends on evaporation. In arid climates, a 12-inch layer of sludge can attain a solids content of 40 to 50% in 30 to 40 days. In the same climate, it might require 100 to 250 days for a 3 feet thick sludge layer to reach 50% solids. The annual evaporation rate from a paved drying bed is given by (USEPA, 1987, Eq. 6-11): EQU R.sub.e =62.4(K.sub.e)(E.sub.p) (1)
where:
This equation may be rewritten as follows using Ke=0.6 and evaporation in inches per year: EQU Re=0.6 Ep (2)
where:
The performance of the paved drying bed may be severely limited by rain falling on the loosened sludge. Because of large voids in the loosened sludge, rainfall will be retained in the sludge except during extremely intense rainfall, after which excess water may be decanted. The proportion of rainfall requiring evaporation would be 70 to 100% of the annual rainfall, which greatly increases the total time to achieve the desired solids content. Therefore, the paved drying bed requires a very large area to treat a large volume of sludges. For example, the City of Forth Worth, Tex. operates a paved bed area of 193 acres for sludge drying, as indicated in USEPA, 1987, Section 6.7.
As an indication of the paved bed performance, Table 1 presents estimated annual net water loss for four regions based on Equation 2, published evaporation/precipitation data (Linsley, et al., 1975, FIG. 3-14 for precipitation and FIG. 5-7 for evaporation), and an assumed rainfall retention rate of 80%. Table 1 indicates that paved bed drying is effective only in arid (Phoenix, Ariz.) and semi-arid (Fort Worth, Tex.) regions. Although the paved bed is not effective in humid climates, it may show limited success during periods of little or no precipitation.
TABLE 1 ______________________________________ Net Water Loss by Paved Drying Beds (in/year) Savannah Madison Fort Worth Phoenix Georgia Wisconsin Texas Arizona ______________________________________ (1) Lake Evap 44 30 60 72 (2) Pan Evap 62 42 84 101 (3) PBD Evap 37 25 50 60 (4) Precipitation 48 32 28 8 (5) Rewetting 38 26 22 6 (6) Net Water 1 -1 28 54 Loss ______________________________________ Notes: (1) Lake Evaporation from Linsley, et al., FIG. 5-7 (2) Pan Evaporation, Ep = Lake Evaporation .times. 1.4 (3) PDB Evaporation from Equation Re = 0.6 Ep (4) Precipitation from Linsley, et al., FIG. 3-14 (5) Rewetting depth = 0.8 .times. Precipitation (6) Net water loss = PDB Evaporation - Rewetting depth
It should be noted that the paved drying bed, like sand bed drying, is a batch operation facility and the dried sludge must be removed prior to the next drying batch is placed. Also, because of the large land area required, the paved bed procedure is primarily applicable to towns or cities with large land areas available at low cost.
The prior art methods presented above are either expensive or ineffective for large volume sludges being generated or accumulated over a long period of time. Thus, sludge-generating industries have yet to find an economical and environmentally safe method for sludge management.